Second-harmonic radiation is generated at a gold surface by use of a laser pulse that is varied in duration from 14 to 29 fs and in intensity from 10<sup>9</sup> to 10<sup>11</sup>W/cm<sup>2</sup> . At laser intensities below 10<sup>10</sup>W/cm<sup>2</sup> , the second-harmonic signal has the expected quadratic dependence on pump-laser intensity; however, at higher intensities, the dependence is supraquadratic. This difference arises because the leading edge of the laser pulse interacts significantly with the gold electrons to create a nonequilibrium, photoexcited distribution. The second-harmonic generation process occurs before electron–electron or electron–phonon collisions can equilibrate the distribution and therefore serves as a probe of the nonequilibrium distribution.
© 1999 Optical Society of America
K. L. Moore and T. D. Donnelly, "Probing nonequilibrium electron distributions in gold by use of second-harmonic generation," Opt. Lett. 24, 990-992 (1999)